In-Situ/In-Operando HX-NEXAFS (Electrochemical; IS-HX-NEXAFS, 5~27 keV)
In-Situ/In-Operando Hard X-ray Near Edge Absorption Fine Structure (HX-NEXAFS) is an advanced spectroscopic technique utilized for real-time analysis of materials undergoing electrochemical processes. Operating within the energy range of 5 to 27 keV, IS-HX-NEXAFS employs hard X-rays to probe the near-edge absorption of elements in situ or in operando conditions. This method provides unique insights into the electronic and chemical changes occurring at the material's surface during electrochemical reactions.
By capturing the fine structure of the X-ray absorption spectrum in real-time, IS-HX-NEXAFS offers detailed information about the evolution of oxidation states, bonding configurations, and surface chemistry of the material under electrochemical conditions. This technique is invaluable for studying electrochemical systems such as batteries, fuel cells, and electrocatalysts, providing a critical understanding of their performance, stability, and mechanisms.
IS-HX-NEXAFS holds significant promise for advancing the development of electrochemical devices and materials by offering insights into their dynamic behavior under operating conditions. It bridges the gap between fundamental understanding and practical applications, paving the way for improved design and optimization of electrochemical technologies for energy storage, conversion, and other applications.
Electrocatalysis
To understand the enhanced activity of Pt/np-Co0.85Se in a neutral electrolyte, in situ and operando Co K-edge XANES and FT-EXAFS spectra were measured during HER working conditions. The electronic structure and local atomic environment changes of np-Co0.85Se and Pt/np-Co0.85Se were probed. In Fig. 1a, as bias voltages increase from OCV to -0.2 V vs. RHE, there is no significant change in the absorption onset of np-Co0.85Se. However, in Pt/np-Co0.85Se (Fig. 1b), a slight shift toward higher energy is observed, particularly evident in the first-order derivatives of the XANES spectra (Fig. 1c, d). This distinct behavior indicates the influence of Pt and Co0.85Se electronic interactions in enhancing electron transfer from Co to Se during HER. Additionally, similar trends in the Co-Se shell radial distance are observed in FT-EXAFS spectra (Fig. 1e, f). The radial distance decreases with applied bias on Pt/np-Co0.85Se (Fig. 1f) but not on np-Co0.85Se (Fig. 1e), suggesting an increase in electron intensities on Pt/Co0.85Se.
Reference:
Jiang K, Liu B, Luo M, Ning S, Peng M, Zhao Y, et al. Single platinum atoms embedded in nanoporous cobalt selenide as electrocatalysts for accelerating hydrogen evolution reaction. Nat Commun 2019;10:1–10.
https:/ /doi.org/10.1038/s41467-019-09765-y.
Fuel Cells
To investigate catalyst degradation in actual PEMFC conditions, continuous fuel-cell operation was conducted on Membrane Electrode Assembly (MEA) for 8 hours at 0.5 V. XANES spectra (Fig. 2) during this period revealed a gradual shift in the Fe K-edge towards higher photon energies, resembling the Fe2(SO4)3 reference spectrum, indicating Fe species dissolution. Linear Combination Fitting (LCF) with initial MEA and Fe2(SO4)3 spectra suggested the demetallation of FeNx centers as a cause for catalyst degradation in the initial hours, with some stability observed in the remaining FeNx centers. Highly coordinated Fe centers like FeN4 may be more tolerant to dissolution. Continuous degradation over 8 hours could involve other mechanisms, such as micropore flooding, highlighting the utility of in situ MEA testing for real-time information on Fe species status during fuel cell operation.
Reference:
Nabae Y, Yuan Q, Nagata S, Kusaba K, Aoki T, Takao N, et al. In Situ X-ray Absorption Spectroscopy to Monitor the Degradation of Fe/N/C Cathode Catalyst in Proton Exchange Membrane Fuel Cells. J Electrochem Soc 2021;168:014513.
https://doi.org/10.1149/1945-7111/abdc64.
Electrochemical Capacitors
In this study, researchers utilized in situ Mn K-edge fluorescence X-ray absorption spectroscopy (XAS) to examine manganese oxides deposited on a porous carbon paper substrate. The aim was to investigate their suitability for electrochemical capacitors, focusing on understanding local and electronic structural changes in response to applied potential in a neutral electrolyte. To accurately determine the oxidation state of Mn at different potentials, XANES spectra at +1.0, +0.8, +0.1, and -0.3 V Vs SCE were compared with reference oxides (MnO, Mn2O3, LiMn2O4, and Mn(IV)O2). The analysis revealed a linear relationship between energy shift and oxidation state. The edge position measured at the first inflection point for reference compounds demonstrated a linear dependence on the formal Mn valence from 2+ to 4+ with a high correlation (R > 0.99), as depicted in Figure 3a. The calculated average oxidation state for Mn at each applied potential is illustrated in Figure 3b, showing values of 4.04, 3.98, 3.71, and 3.10 at +1.0, +0.8, +0.1, and -0.3 V Vs SCE, respectively. This analysis indicated that the average oxidation state of Mn ranged from 3.71 to 3.98 during the electrochemical redox reaction within the potential range of +0.1 to +0.8 V Vs SCE. This range corresponds to the reversible region where ideal capacitive behavior was observed, suggesting the electrochemical utilization of 21% of the Mn sites in manganese oxide.
Reference:
Nam KW, Kim MG, Kim KB. In situ Mn K-edge X-ray absorption spectroscopy studies of electrodeposited manganese oxide films for electrochemical capacitors. J Phys Chem C 2007;111:749–58.
https://doi.org/ 10.1021/jp063130o.
Degradation of Electrocatalyst
In this study, Friebel et al. investigated the anodic oxidation of small platinum (Pt) islands supported on single-crystal rhodium (111) and gold (111) substrates. The research employs in situ X-ray absorption spectroscopy (XAS) in the high-energy resolution fluorescence detection (HERFD) mode. The in situ HERFD XAS measurements on Pt/Rh(111) (Figure 4a) and Pt/Au(111) (Figure 4b) reveal notable alterations in the white-line region as the potential exceeds 1.0 V. Initially, on both samples, there is a shift from a narrow absorption peak at 11566 eV to a broader peak around 11567 eV, resembling changes observed in a previous study of 2D Pt/Rh(111). However, while this new feature plateaus on both 2D Pt/Rh(111) and 3D Pt/Rh(111) with further potential increase, a secondary transition occurs on Pt/Au(111) after reaching 1.4 V. At this potential, there is a significant increase in white-line intensity over approximately 40 minutes, and the absorption peak shifts to 11568 eV. In summary, our XAS findings indicate that anodic Pt dissolution is enhanced on an Au(111) substrate, whereas anodic polarization of Pt/Rh(111) results in passivation. Consequently, we anticipate that Pt/Rh(111) or Pt/Rh nanoparticles will exhibit robust long-term stability under oxygen reduction reaction (ORR) conditions.
Reference:
Friebel D, Miller DJ, Nordlund D, Ogasawara H, Nilsson A. Degradation of bimetallic model electrocatalysts: An in situ x-ray absorption spectroscopy study. Angew Chemie - Int Ed 2011;50:10190–2.
https://doi. org/10.1002/anie.201101620.